According to patent document 1, it is proposed to form an intermediate layer with a low hardness on a sapphire substrate, to form a seed crystal substrate of gallium nitride thereon and to grow a thick film of gallium nitride by the flux method. The thick film of gallium nitride is separated from the sapphire substrate during the cooling step.
Further, according to patent document 2, when a seed crystal film is formed on a sapphire substrate, voids are formed along an interface of the sapphire substrate and seed crystal film. Then, during the cooling step after a thick film of gallium nitride is formed on the seed crystal film, the thick film of gallium nitride is spontaneously separated from the sapphire substrate.
According to patent document 3, when a seed crystal film is formed on a sapphire substrate, voids are formed along an interface between the sapphire substrate and seed crystal film and the void ratio is adjusted at a value of 12.5 percent or lower. Cracks or defects in gallium nitride crystal grown on the seed crystal film are thereby reduced.